System for dealing with faults in an optical link

Abstract

A system for dealing with faults in wavelength division multiplexed (WDM) optical communications between two terminals connected by at least two optical fibers monitors the status of communications over both optical fibers. If both optical fibers are operating normally, a first set of channels is routed over the first optical fiber and a second set of channels (which is mutually exclusive of the first set of channels) is routed over the second optical fiber. However, if a fault is detected in either optical fiber, the first terminal combines the first and second sets of channels and routes the combined channels over the remaining optical fiber to the second terminal. The second terminal separates the combined channels to recreate the first and second sets of channels. Wavelength slicers can be used to multiplex and demultiplex the channels at both terminals. This architecture allows the first and second sets of channels to be interdigitally spaced.

We claim: 
     
       1. A method for optical communications from a first terminal to a second terminal over a first optical fiber and a second optical fiber using wavelength division multiplexed (WDM) signals containing a plurality of channels, said method comprising: monitoring the status of said first optical fiber to define a normal state in which the first optical fiber is operating normally, and a fault state in which the first optical fiber has a fault;   in said normal state, routing a first set of channels over the first optical fiber and routing a second set of channels over the second optical fiber, wherein said first set of channels and said second set of channels are mutually exclusive; and   in said fault state: (a) combining said first and second sets of channels at the first terminal;   (b) routing the combined channels over the second optical fiber to the second terminal; and   (c) separating the combined channels at the second terminal into said first and second sets of channels.     
     
     
       2. The method of claim 1 wherein said first set of channels and said second set of channels are interdigitally spaced. 
     
     
       3. The method of claim 1 wherein said step of combining said first and second sets of channels at said first terminal is performed by at least one wavelength slicer. 
     
     
       4. The method of claim 1 wherein said step of separating said combined channels at said second terminal is performed by at least one wavelength slicer. 
     
     
       5. A method for optical communications from a first terminal to a second terminal over a first optical fiber and a second optical fiber using wavelength division multiplexed (WDM) signals containing a plurality of channels, said method comprising: monitoring the status of the said first optical fiber and said second optical fiber to define a normal state in which the first optical fiber is operating normally, a first fault state in which the first optical fiber has a fault, and a second fault state in which the second optical fiber has a fault;   in said normal state, routing a first set of channels over the first optical fiber and routing a second set of channels over the second optical fiber, wherein said first set of channels and said second set of channels are mutually exclusive;   in said first fault state: (a) combining said first set of channels and said second set of channels at the first terminal;   (b) routing the combined channels over the second optical fiber to the second terminal; and   (c) separating the combined channels into said first and second sets of channels at the second terminal; and     in said second fault state: (a) combining said first set of channels and said second set of channels at the first terminal;   (b) routing the combined channels over the first optical fiber to the second terminal; and   (c) separating the combined channels into said first and second sets of channels at the second terminal.     
     
     
       6. The method of claim 5 wherein said first and second sets of channels are interdigitally spaced. 
     
     
       7. The method of claim 5 wherein said step of combining said first and second sets of channels at said first terminal in said fault states is performed by at least one wavelength slicer. 
     
     
       8. The method of claim 5 wherein said step of separating said combined channels at said second terminal in said fault states is performed by at least one wavelength slicer. 
     
     
       9. An optical communications system providing optical communications from a first terminal to a second terminal over at least a first optical fiber and a second optical fiber carrying wavelength division multiplexed (WDM) signals containing a first set of channels and a second set of channels, wherein said first and second sets of channels are mutually exclusive, and wherein said optical communications system comprises: a controller monitoring the status of said first and second optical fibers, said controller defining a normal state in which both of said first and second optical fibers are operating normally, a first fault state in which the first optical fiber has a fault, and a second fault state in which the second optical fiber has a fault;   said first terminal having: (a) a first multiplexer combining optical signals from a first input port and a second input port, wherein said first input port receives said first set of channels;   (b) a second multiplexer combining optical signals from a first input port and a second input port, wherein said second input port receives said second set of channels;   (c) a first optical switch selectively routing the combined optical signals from said first multiplexer to said first optical fiber in the normal state and the second fault state, and to said first input port of said second multiplexer in the first fault state; and   (d) a second optical switch selectively routing the combined optical signals from said second multiplexer to said second optical fiber in the normal state and the first fault state, and to said second input port of said first multiplexer in the second fault state; and     said second terminal having: (a) a first demultiplexer separating optical signals from an input port into a first set of channels at a first output port and a second set of channels at a second output port;   (d) a second demultiplexer separating optical signals from an input port into a first set of channels at a first output port and a second set of channels at a second output port;   (c) a third optical switch selectively routing the channels from the first optical fiber to said input port of said first demultiplexer in the normal state and the second fault state, and selectively routing said first set of channels from said first output port of said second demultiplexer to said input port of said first demultiplexer in the first fault state; and   (d) a fourth optical switch selectively routing the channels from the second optical fiber to said input port of said second demultiplexer in the normal state and the first fault state, and selectively routing said second set of channels from said second output port of said first demultiplexer to said input port of said second demultiplexer in said second fault state.     
     
     
       10. The optical communications system of claim 9 wherein said first and second sets of channels are interdigitally spaced. 
     
     
       11. The optical communications system of claim 9 wherein at least one of said first multiplexer, second multiplexer, first demultiplexer, and second demultiplexer comprise a wavelength slicer. 
     
     
       12. The optical communications system of claim 11 wherein said wavelength slicer comprises: a first polarization-dependent routing element spatially separating an input beam containing a plurality of channels into a pair of orthogonally-polarized beams;   a polarization rotator for rotating the polarization of at least one of said pair of orthogonally-polarized beams to create first and second beams having the same polarization;   a wavelength filter coupled to receive said first and second beams from said polarization rotator, said wavelength filter having a polarization-dependent optical transmission function such that said first beam decomposes into third and fourth beams with their polarizations orthogonal to each other, and said second beam decomposes into fifth and sixth beams with their polarizations orthogonal to each other, wherein said third and fifth beams carry said first set of channels at a first polarization and said fourth and sixth beams carry said second set of channels at a second polarization, wherein said first and second polarizations are orthogonal; and   a second polarization-dependent routing element spatially routing said third and fifth beams carrying said first set of channels along a first optical path and said fourth and sixth beams carrying said second set of channels along a second optical path based on their polarizations.   
     
     
       13. The optical communications system of claim 12 wherein said wavelength filter comprises a stacked plurality of birefringent waveplates with each waveplate oriented in a predetermined direction. 
     
     
       14. An optical communications system providing optical communications from a first terminal to a second terminal over a first optical fiber and a second optical fiber carrying wavelength division multiplexed (WDM) signals containing a first set of channels and a second set of channels that are interdigitally spaced between said first set of channels; said optical communications system comprising: a controller monitoring the status of said first and second optical fibers, said controller defining a normal state in which both of said first and second optical fibers are operating normally, a first fault state in which the first optical fiber has a fault, and a second fault state in which the second optical fiber has a fault;   said first terminal having: (a) a first wavelength slicer combining optical signals from a first input port and a second input port, wherein said first input port receives said first set of channels;   (b) a second wavelength slicer combining optical signals from a first input port and a second input port, wherein said second input port receives said second set of channels;   (c) a first optical switch selectively routing the combined optical signals from said first wavelength slicer to said first optical fiber in the normal state and the second fault state, and to said first input port of said second wavelength slicer in the first fault state; and   (d) a second optical switch selectively routing the combined optical signals from said second wavelength slicer to said second optical fiber in the normal state and the first fault state, and to said second input port of said first wavelength slicer in the second fault state; and     said second terminal having: (a) a third wavelength slicer separating optical signals from an input port into a first set of channels at a first output port and a second set of channels at a second output port;   (d) a fourth wavelength slicer separating optical signals from an input port into a first set of channels at a first output port and a second set of channels at a second output port;   (c) a third optical switch selectively routing the channels from the first optical fiber to said input port of said third wavelength slicer in the normal state and the second fault state, and selectively routing said first set of channels from said first output port of said fourth wavelength slicer to said input port of said third wavelength slicer in the first fault state; and   (d) a fourth optical switch selectively routing the channels from the second optical fiber to said input port of said fourth wavelength slicer in the normal state and the first fault state, and selectively routing said second set of channels from said second output port of said third wavelength slicer to said input port of said fourth wavelength slicer in said second fault state.     
     
     
       15. The optical communications system of claim 14 wherein at least one of said wavelength slicers comprise: a first polarization-dependent routing element spatially separating an input beam containing a plurality of channels into a pair of orthogonally-polarized beams;   a polarization rotator for rotating the polarization of at least one of said pair of orthogonally-polarized beams to create first and second beams having the same polarization;   a wavelength filter coupled to receive said first and second beams from said polarization rotator, said wavelength filter having a polarization-dependent optical transmission function such that said first beam decomposes into third and fourth beams with their polarizations orthogonal to each other, and said second beam decomposes into fifth and sixth beams with their polarizations orthogonal to each other, wherein said third and fifth beams carry said first set of channels at a first polarization and said fourth and sixth beams carry said second set of channels at a second polarization, wherein said first and second polarizations are orthogonal; and   a second polarization-dependent routing element spatially routing said third and fifth beams carrying said first set of channels along a first optical path and said fourth and sixth beams carrying said second set of channels along a second optical path based on their polarizations.   
     
     
       16. The optical communications system of claim 15 wherein said wavelength filter comprises a stacked plurality of birefringent waveplates with each waveplate oriented in a predetermined direction.